This invention relates in general to eliminating one-per-rev vibratory forces in rotary wing aircraft and more particularly to sensing the magnitude and span or chord inertial or aerodynamic direction of each of a plurality of unbalanced radial forces and bending moments acting on a helicopter mast to correct the cause of the unbalance. The invention more specifically is addressed to force tracking helicopter rotors.
Vibration is generally undesirable in rotating machinery. This is particularly true in helicopters. Not only does vibration cause excessive noise and physical discomfort to passengers and crew, but it also causes joints and fasteners to loosen and can contribute to fatigue stress failures in vital components. Consequently, it is important for safety and comfort that helicopter vibration be minimized.
Rotating helicopter blades comprise a primary source of helicopter vibration. A first cause of vibration may be that the blades are out of inertial balance, i.e., the center of gravity is displaced from the mast axis. When such blades rotate, a radial force having a frequency of one cycle per-revolution acts perpendicularly to the axis of the mast and is of magnitude proportional to the unbalance.
A second cause of vibration is aerodynamic unbalance. This unbalance is caused by the center of lift being displaced from the center of rotation and/or by the rotor blades having different drag characteristics. The resultant forces act on the mast. In such a case, the center of rotation of the blades wobbles about the mast axis. A bending moment is transmitted by the mast through its bearings to the rotor pylon where reaction forces cause vibrations of a one-per-revolution frequency in the airframe.
Inertial unbalance produces a radial force at the mast hub. Aerodynamic unbalance produces a bending moment and/or a radial force about the mast hub. Both result in forces which produce undesired vibration in the frame of the helicopter. The problem is to isolate the causes of unbalance in the environment of a complex set of forces and eliminate or compensate them.
Likelihood of inertial unbalance can be reduced by careful manufacturing and assembly procedures in which the rotor blades are matched for weight to locate the center of gravity at the center of the mast so that the centrifugal forces produced by the blades are as equal as possible. To the extent that manufacturing and assembly techniques are not perfect, a residual inertial unbalance condition can remain.
Some of the factors which cause aerodynamic unbalance can also be minimized during manufacture and assembly by careful match in the airfoil shapes of each blade and careful alignment of the individual blade pitch and sweep angle adjustment mechanisms. With blades matched and properly assembled, subsequent warping and play in the pitch and sweep angle adjustment linkages may result in substantial unbalance.
Prior methods for correcting rotor unbalance have several disadvantages, among which are that they are indirect and approximate. They are based upon a questionable assumption that all forces will be in balance when the blades geometrically track in the same plane.
One purpose of this invention is to provide a response to unbalanced radial forces acting on rotating helicopter masts and to define the sources causing the unbalance and to indicate the magnitude and direction of the compensating change necessary to minimize the vibration.
Further, there is provided a continuous monitor of the force track of helicopter rotor blades to aid in making corrections necessary to minimize vibrations from aerodynamic and inertial unbalance of the rotor assembly.
It is a still further object of the invention to provide a helicopter rotor force track sensing device which is accurate, reliable, simple and relatively inexpensive.
It is another object of the invention to provide in a force track sensing device of the character described, an output signal which is adapted either for visual display or for use in a servo control system for automatically correcting out of selected balance conditions.